The present invention generally relates to a optical modulator having superior 3 rd-order nonlinear optical performance. More specifically, the present invention is directed to an organic/inorganic composite superlattice type optical modulator suitable for an optical communication, and an optical information processing operation with employment of a large-capacity light pulse stream, and a response speed on the order of sub-picosecond, and also low switching power. The large-capacity light pulse stream owns a pulse time width less than several tens of picoseconds.
In connection with rapid development in current multimedia fields, a strong demand is made in order to transmit/receive large amounts of information through large-capacity highspeed optical communications.
Presently, electric signals transmitted via telephone lines from information originating sources such as a plurality of homes and offices are collected to a repeater station operated for a long distance communication. In this repeater station, these electric signals are converted into light signals. Then, a large number of light signals are transmitted from this repeater station via optical fibers to another repeater station located apart from the first-mentioned repeater station by approximately several hundreds Km (kilometers). In this repeater station, these light signals are again converted into electric signals which will be sent to a target information receiving source.
A strong need is made as to not only an increase in the quantity of information originating sources for utilizing such a optical communication, but also large data capacities (e.g., computer data files and computer graphics) and highspeed data communications in addition to simple speech (voice) information.
Furthermore, to realize more highspeed communications, electric signals produced from all of information originating sources are gradually, optically processed to emit light signals. To this end, shorter light pulses are employed so as to increase a signal transmission amount per unit time.
Optical fibers used in such a light signal transfer operation are selected from optical fibers made of mainly silica glass. Thus, light having a wavelength of 1.3 .mu.m, or 1.5 .mu.m is utilized as signal light, under which the light transfer loss of this silica glass fiber is minimized.
For instance, the light transfer loss occurred in this wavelength of 1.3 .mu.m, or 1.5 .mu.m is on the order of 0.5 dB/Km. When the wavelength is slightly shifted, the resultant light transfer loss will exceed 1 dB/Km. As a consequence, as a light source used in an optical communication, such laser light with superior monochromaticity fitted to a characteristic of this optical fiber is employed. The intensity of this specific laser light is varied by an optical modulator in response to an electric signal providing information, and thus a light pulse stream is produced to form a desirable light signal.
Even when such a light pulse is used to carry out in an optical communication, if this light pulse is transferred over a line of several hundreds Km, then this light pulse is attenuated. Accordingly, a repeater for amplifying the light pulse is employed in a halfway of this long line.
In a repeater used now, a light signal received by the repeater is converted to an electrical signal by an optical detector to execute electrically decoding processings of amplification, reproducing, retiming and so on, so that a laser light is modulated by such the electrical signal and transmitted to a next station as a light signal.
As explained above, the following demodulating technique is required. That is, the light signal which has reached in the form of such a highspeed light signal is directly demodulated in the repeater without executing the electric process operations. This type of highspeed light modulation constitutes a necessary requirement in the high-capacity highspeed communication operable at the speed higher than several tens G bit/sec, which implies a limit in an electric signal processing operation. Therefore, light modulations by light with higher speeds are required.
When a light modulation is carried out by light, 4-light-wave mixing and an optical bistable phenomenon, and the like are employed which correspond to one of 3rd nonlinear optical effects of a substance. They are caused by a change in a nonlinear refractive index. Normally, when light is irradiated onto a substance, polarization is induced into the substance in direct proportion to a magnitude of a light electric field thereof.
A "nonlinear optical effect" implies all of such effects among polarization of a substance, which is induced when light is entered into the substance, that the polarization is directly proportional to an incident light electric field. The effects which are directly proportional to the second power of light electric field, the third power of light electric field, . . . , are referred to as a 2nd-order nonlinear optical effect, a 3rd-order nonlinear optical effect, . . . (see "The Elements of Nonlinear Optics" written by P. N. Butcher, and D. Cotter, Cambridge Present Optical Research 9, Cambridge University Publisher, 1990).
The polarization "P" caused by the 3rd-order nonlinear optical effect is expressed by the following formula [1] in the case that the substance owns the central symmetry: ##EQU1##
In the above-described formula [1], symbol "t" indicates time, symbol ".omega." represents an angular frequency, symbol ".di-elect cons..sub.0 " denotes a dielectric constant (permittivity) in vacuum, and symbol "E.sub.107.sup.(t) " shows an incident light electric field. Also, symbol ".chi..sup.(1) " indicates a linear susceptibility, and is defined by a linear refractive index "n.sub.0 " of a substance, and the following formula [2]: ##EQU2##
In the above-explained formula [2], symbol "Re" indicates that this value takes a real part of the linear susceptibility .chi..sup.(1). Also, symbol ".chi..sup.(3) " indicates 3rd-order nonlinear susceptibility of a substance, and is defined by a 3rd-order nonlinear refractive index "n.sub.2 " of a substance, and the following formula [3]: ##EQU3##
As apparent from these formulae, a refractive index n () of a substance may be expressed by the below-mentioned formula [4] based upon the linear refractive index "n.sub.0 " and the non-linear refractive index "n.sub.2 " : EQU n(.omega.)=n.sub.0 (.omega.)+n.sub.2 (.omega.).vertline.E.sub..omega. (t) .vertline..sup.2 =n.sub.0 (.omega.)+n.sub.2 (.omega.)I [4]
In the above-mentioned formula [4], symbol "I" shows intensity of light. This reflects such a fact that when the intensity of light is low, the refractive index of the substance is seemed that this refractive index becomes a constant value "n.sub.0 " irrespective of the intensity of the incident light, whereas while intensity of strong light such as laser light is increased, the refractive index of the substance is changed.
As previously described, since a substance constant is modulated by such strong intensity light, physical characteristics of a refractive index, an absorption factor, polarization of light, and a phase of a substance are varied. As a result of these variations, a direction and intensity of light which passes through, or is reflected by this substance, may be changed.
Since such a light modulation is induced by a light electric field by incident light, polarization of a substance occurs in a speed of light, and physical characteristics thereof may be rapidly changed. However, the polarization once induced in the substance remains for a preselected time duration after the incident light has passed, and this time duration differs to each other, depending upon polarization mechanisms as substances.
For example, in a semiconductor such as "GaAs" and "InSb", excitons are produced which are separated into electrons and holes due to optical pumping. A time duration longer than several nanoseconds (10.sup.-9 seconds) is required until these excitons are recombined with each other to be returned to original states. The reason is given as follows. Since the atoms for constituting the semiconductor are combined with each other in the regular manner due to the covalent bond by atoms, both the electrons and the holes separated from these electrons are travelled over a plurality of atoms, and therefore the electrons are located far from the holes.
To the contrary, as to organic molecule such as polydiacetylene and metal phthalecyanine, even when the molecules are brought into an excitation condition by optical pumping, since there is no covalent bond between the adjoining molecule, both electrons and holes are not separated from each other, but this excitation state is disactivated, so that only such a short time duration shorter than several picoseconds (10.sup.-12 seconds) until these molecules under excitation state are returned to the original state.
On the other hand, if the electron charge amount of a polarized electrons is made equal to that of the polarized holes, then since the magnitude of the polarization of such a substance is directly proportional to the separated distance, there is a trade-off relationship between the magnitude of the polarization and the speed of the disactivation.